Asymmetrical hydrogenation

#The present invention relates to an asymmetrical hydrogenation, more specifically to a process for the preparation of compounds of formula ##STR3## wherein R.sub.1 is hydrogen or a group Z, where ##STR4## wherein R.sub.2 is methyl, ethyl or chloromethyl, and R.sub.3 is C.sub.1 -C.sub.4 alkyl, or R.sub.2 and R.sub.3, together with the linking carbon atom, are C.sub.3 -C.sub.7 cycloalkyl.

#The process of this invention comprises asymmetrically hydrogenating a compound of formula ##STR5## wherein R.sub.1 is as defined above.

#The asymmetrical hydrogenation is conveniently carried out using a neutral or cationic rhodium complex of a chiral atropisomeric phosphine. In particular, the hydrogenation can be carried out in the presence of a rhodium catalyst of formula

#Within the scope of this invention, the term "neutral ligand" may be taken to mean a readily exchangeable ligand such as an olefin, including ethylene, propylene, cyclooctene, 1,5-hexadiene, norbornadiene, 1,5-cyclooctadiene and the like, a nitrile such as acetonitrile or benzonitrile, or also the solvent employed and the like. This ligand can be replaced in the course of the hydrogenation. If more than one such ligand is present, they may also differ from one another.

#Surprisingly, it has now been found that, compared with the known catalysts for such purposes, the rhodium diphosphine complexes of formulae V and VI are appreciably more active and enantioselective. This means in particular that substantially smaller amounts of catalyst can be used, that shorter reaction times are possible, and that optical yields (e.e.) of more than 95% can be obtained.

#The asymmetrical hydrogenations can be carried out in suitable organic solvents which are inert under the reaction conditions. Such solvents are typically aromatic hydrocarbons such as benzene or toluene, cyclic ethers such as tetrahydrofuran or dioxane, esters, such as ethyl acetate, or also mixtures thereof, and the like. The ratio of rhodium to ligand Y is conveniently from about 0.05 to about 5 mol, preferably from about 0.5 to about 2 mol, of rhodium per mol of ligand. The ratio of rhodium to the radical X is from about 0.01 to about 20, preferably from about 0.5 to about 10 mol of rhodium per mol of radical X. The molar ratio of rhodium in the complexes of formulae V and VI to the compounds of formula II to be hydrogenated is conveniently from about 0.001 to about 5 mol %, preferably from about 0.002 to about 0.02 mol %.